This invention relates generally to means for improving the service-life and reliability of indirect heat exchangers used in chemical reaction systems which produce high temperature, reactive effluent gasses. In particular, this invention relates to means for improving service-life of indirect heat exchangers used in the production of hydrogen cyanide.
In many chemical processes, the reaction effluent comprises hot, reactive, and/or abrasive fluids and/or gasses. For many of these reactions, it is desirable to rapidly quench the reactor effluent to prevent decomposition of the product component(s). Quenching may be accomplished through direct contact, such as application of water sprays, or more commonly through indirect methods such as through the use of indirect heat exchangers. Because indirect heat exchangers provide the added advantage that they may be configured to recover waste heat, these are the more preferred method and have been in use for many years. Typical indirect heat exchangers used in such chemical processes consist of a shell and tube design.
In many systems, the thermal, kinetic and reactive properties of the effluent may individually or collectively serve to corrode, crack, or otherwise degrade the materials used to form the heat exchange zone. In particular, the exchange tubes and the tube-to-tubesheet welds nearest to the reaction zone see the most severe conditions and are most susceptible to degradation. For example, the hot effluent may chemically react with the metal of the heat exchange tube itself, thereby causing erosion and/or corrosion, that is, metal dusting, carburization and the like, all of which lead to failure of the heat exchanger. The weld area of the heat exchange tubes is also susceptible to stress corrosion cracking, which leads to failure of the heat exchanger. These problems may be encountered, for example, in the production of hydrogen cyanide or acrylonitrile, in nitric acid waste heat recovery exchangers, in hydrocarbon cracking units, and in tubeside fired boilers and exchangers.
The use of ferrules as a protective covering for tubes in shell and tube heat exchangers and condensers is well known. Such ferrules primarily insulate the tubes and welds but also protect the heat exchange tubes against deterioration resulting from chemical attack. Typically, heat exchangers are placed downstream of a reaction zone in a chemical reactor, such as downstream of a catalyst. Thus, the top or upper portion of the heat exchange tubes are exposed to hot effluent gasses. Certain ferrules, such as alumina, are employed to provide insulation against the heat from such gasses.
In a common application, an exchanger tube is first constructed from a base material; a second, possibly different material, selected for its properties, such as resistance to erosion and/or chemical attack, is then formed into a tube that slides inside the inlet/upstream end of the heat exchanger tube. Depending on the severity of the process conditions, the ferrules may provide a long period of maintenance-free service or alternatively, they may be sacrificial, requiring frequent replacement. In either case, the use of the ferrules provides an economical method for extending the service life of the base-material exchanger through the prevention of erosion and/or corrosion.
For example, M. James, “Unexpected Metal Dusting Failure of Waste Heat Boiler Tubes”, First International Symposium on Innovative Approaches for Improving Heat Exchanger Reliability, Proceedings, Materials Technology Institute of the Chemical Process Industries, Inc., 1-13(1998) discloses a specific ceramic ferrule design in a shell and tube reactor for use in a chemical process having reducing, carburizing and/or nitridizing conditions. In this paper, various nickel-chromium alloys (INCONEL) were used in the tubes of a heat exchanger. In each case, the INCONEL material experienced severe wastage, that is, metal dusting, at rates much higher than known rates.
Also, U.S. Pat. No. 5,775,269 discloses a boiler protection tube assembly having an inner ceramic sleeve, a ceramic block and an outer ceramic sleeve. The ceramics disclosed in U.S. Pat. No. 5,775,269 are aluminum and zirconium oxides. Such ceramics are impractical for use in heat exchangers requiring rapid quenching of very hot effluent as these materials last only a relatively short time under such temperature extremes.
Ceramics, such as alumina, silica and zirconia, are effective as insulators in such reactors as steam-methane reformers. However, they suffer from poor thermal shock resistance and may, in the case of silica, react with hydrogen, which is present in many reducing environments. See, for example, M. S. Crowley, Hydrogen-Silica Reactions in Refractories, Ceramic Bulletin, Vol. 46, No. 7, 679-682 (1967). Thus, these ceramic materials are unsuitable for use in chemical processes requiring rapid quenching of hot effluent gasses and/or in reducing environments, both of which are found in the production of hydrogen cyanide.
In a typical shell and tube reactor, the heat exchange tubes are attached to a tubesheet at each tube end. Typically, a tube is passed through a hole in the tubesheet until the end of the tube is approximately flush with the top surface of the tubesheet. The tube is then typically welded to the top surface of the tubesheet. Generally, the outer diameter of the tube is smaller than the inner diameter of the corresponding hole in the tubesheet. Thus, once the tube is welded to the tubesheet, an annular space remains between the tube and the tubesheet below the weld. FIG. 1a shows a typical weld 3 used in shell and tube reactors, especially reactors used for chemical processes having reducing environments, such as in the production of hydrogen cyanide. The tubes may also be affixed to the tubesheet by alternate means, such as rolling. FIG. 1b shows a typical attachment of a tube to a tubesheet by rolling.
In hydrogen cyanide production, the hot effluent gasses must be rapidly cooled from about 1000° to 1400° C. to about 600° C. or less in order to prevent decomposition of the hydrogen cyanide. As such effluent gasses are cooled, the tubesheet, weld and upper portion of the exchange tubes become very hot. As a result, any water present in the annular space 5 vaporizes and deposits any impurities contained in the water in the annular space 5. Such impurities typically are ions and minerals in the water and the like. Such impurities are also typically corrosive to the tubesheet, exchange tube, and particularly the weld. Over time, such corrosive materials buildup in the annular space 5. The combination of heat from the effluent gasses and corrosive materials with stresses in the system leads to stress corrosion cracking in the tube, weld and/or tubesheet. Such stress corrosion cracking leads to failure, and ultimately replacement, of the heat exchanger.
A number of shell and tube hydrogen cyanide reactor designs have been developed to address the problem of minimizing the heat the tubesheet, exchange tubes and weld are exposed to. FIGS. 3a-e illustrate such reactors. Each reactor is designed with high cooling water flow rates, turbulent cooling water flow, and refractory to insulate the tubesheet. However, these designs do not completely prevent such stress corrosion cracking.
Down-hole welds, or full penetration welds, have been used in chemical processes that do not have reducing environments or such rapid quenching requirements. For example, Ahmed et al., Failure, Repair and Replacement of Waste Heat Boiler, Ammonia Plant Safety & Related Facilities, American Institute of Chemical Engineers, Vol. 37, 100-110, discloses the use of such a weld in a horizontal heat exchanger for use in a secondary ammonia reformer. This paper does not disclose the use of this weld for any other process.
Thus, the problem of providing effective heat exchangers having a long-service life in shell and tube reactors used in chemical processes having reducing environments remains.